By Ken Lister, SCS Engineers
June 25, 2007 -- Municipal solid waste (MSW) landfill impacts to groundwater generally have one of two sources: leachate or landfill gas (LFG). Traditional thinking has been that the greatest threat to groundwater posed by landfills is the migration of liquids that come in contact with waste, also known as leachate.
It is now becoming clear that many groundwater quality impacts at landfills are due to migration of LFG. This is particularly true for facilities located in arid and semi-arid regions, where the volume of leachate generated and the vertical distance to groundwater below waste can significantly reduce the probability of leachate migration to groundwater.
Federal regulations promulgated under Resource Conservation and Recovery Act (RCRA), Subtitle D, require groundwater monitoring at nearly all MSW landfills. Guidelines for the design of monitoring programs have been developed at the federal level and by most states. These guidelines tend to be very general in nature and provide little or no advice for the facility operator regarding interpretation of monitoring data. Interpretation is critical to determining the best corrective action approach.
Often the first indication of water-quality impacts is the presence of relatively low concentrations, perhaps in the parts-per-billion range, of volatile organic compounds (VOCs) in samples from perimeter groundwater monitoring wells. Common VOCs detected in groundwater at MSW facilities include vinyl chloride, tetrachloroethene, trichloroethene, 1,1-dichloroethane, 1,1-dichloroethene, dichlorodifluoromethane, benzene and toluene. Since many of these substances are also commonly detected in both LFG and leachate, the mere presence of any of these in water samples is not specifically indicative of the source. It should be noted, however, that LFG migration can impact groundwater hydraulically upgradient of the facility, whereas leachate migration normally can not.
Clues to the VOC source mechanism can often be uncovered by examining inorganic groundwater monitoring data. How concentrations of major ions (calcium, magnesium, sodium, potassium, chloride, sulfate, carbonate, bicarbonate) and certain metals have changed with time is of particular interest. LFG is made up principally of methane and carbon dioxide. While methane has limited solubility in water, carbon dioxide is easily soluble in groundwater and has the capacity to dramatically change the geochemical makeup of this water due to interactions with host minerals in the aquifer.
Dissolution of carbon dioxide in water usually results in formation of bicarbonate or carbonate ions in groundwater (the dominant ion that forms is pH dependent). This process also will effect pH and/or the concentration of other ions in solution. Interactions of water effected by dissolved carbon dioxide with minerals containing calcium and magnesium in the host geologic material can increase the dissolved phase concentration of these substances.
Other changes can occur as well. If the vadose zone atmosphere under or near the landfill contains a high proportion of LFG, the proportion of oxygen will be decreased. The result can affect aquifer oxidation-reduction potential; this can, in turn, affect the solubility of trace metals such as iron and manganese.
Certain inorganic effects are characteristic of LFG impacts to groundwater, although these vary considerably depending on the initial chemistry of the water. Increases in alkalinity, calcium and magnesium are common in LFG influenced groundwater. Certain VOCs, such as those mentioned above, are also indicative of LFG impacts. Results from the perimeter gas monitoring system can also provide important clues regarding source, particularly with regard to localizing the problem areas.
On the other hand, increases in certain inorganic ions are more characteristic of leachate effects, or of influences that are unrelated to the landfill. Certain VOCs are more characteristic of leachate releases than of LFG, including tetrahydrofuran, acetone, and other ketones. Additionally, an increase in chloride or chemical oxygen demand may in some cases be an indication of leachate.
The concentrations of other commonly monitored substances may be effected by leachate but the effects are highly dependant on the original geochemistry of the aquifer. Groundwater parameters whose concentration may be effected by leachate, but also may be influenced by other factors, include ammonia, total organic carbon, sulfate, potassium, iron, and manganese.
If the source of groundwater VOCs can be attributed to migration of LFG, the most effective control may be installation or upgrading of the facility's gas control system. Active gas control, with an adequate number and spacing of extraction points, will often control LFG-sourced groundwater impacts. Improvement of landfill cover often is proposed as a part of the corrective action for leachate impacts to groundwater, however reducing cover permeability to limit water infiltration can actually worsen the LFG situation by reducing the amount of gas that migrates out of the fill through the cover. Changes in cover characteristics may need to be accompanied by upgrades to the LFG control system.
Another factor that may be worth examining includes the design of the groundwater monitoring system. Monitoring wells may be placed too close to the fill to be unaffected by LFG impacts under even normal circumstances. Detection monitoring wells may not be placed in locations directly downgradient of the fill. Long well screens that extend above the groundwater interface may themselves provide an avenue for migrating LFG to impact groundwater. Optimization of the groundwater monitoring system may be a significant aspect of corrective action.
Ken Lister is a senior technical manager with SCS Engineers in Long Beach, Calif. Since 1970, SCS Engineers has delivered economically and environmentally sound solutions for solid waste management and site remediation projects throughout the world. The award-winning firm provides engineering, construction, and long-term operations and maintenance services to private and public sector clients through a network of 40 offices in 17 states.